High vacuum vapor deposition is used for the production of thin film layers for many industrial applications. In the optics field, sophisticated coating designs consisting of up to 100 single layers are employed in the production of multi-layer interference filters and antireflection coatings for use with prescription and sunglass lenses.
Evaporants are metals, alloys and inorganic compounds generally named “dielectrics”. Metals and alloys are “absorbing materials” and only in a very low thickness (few nanometers) are “semireflective”. Dielectrics are transparent and, depending on the stack of layers employed, enhance various reflected colors (wave length). Stacks are formed by alternating layers of high and low refractive index dielectrics with different optical thickness. With a proper choice of evaporants, film thickness and number of transparent films, it is possible to design numerous types of optical thin films based on interference that meet precise performance demands with a variety of reflections.
Multilayer, antireflection and mirror coatings are commonly applied on a variety of ophthalmic and sunglass lenses, not only for aesthetic purposes, but also because in some cases they can increase lens performance. For example, an antireflection treatment on ophthalmic lenses and a mirror coating on sunglass lenses for high mountain sports can enhance lens performance.
Photochromic lenses, however, are not as compatible with these treatments. The treatment on the outer surface of photochromic lenses can significantly impede photochromic activity and inhibit darkening of the lenses when the lenses are exposed to sunlight.
Photochromic lenses are lenses that adapt their transmission to the variable light intensity while keeping the brightness on the eyes sufficiently constant. Photochromic lenses are useful when a big change of luminous intensity take place, for example, when getting out of the car and into the sunshine, or during open air activities such as activities on high mountains and at seaside resorts.
Photochromic lenses exhibit a reversible change in transmission when exposed to a light radiation involving ultraviolet rays, such as the ultraviolet radiation in sunlight or the light of a mercury lamp. Photochromic lenses can be made of glass or plastic, both of which work through the same principle: when they absorb specific wavelengths around the UVA (ultraviolet) range from about 315 to 380 nm, the photochromic molecules contained in the lens change their status and consequently cause the lens to darken.
These photochromic lenses are generally sold without any vacuum treatment on the surface because vacuum treatments generally absorb a significant percent of UVA radiation, which is responsible for the lens darkening. The most commercially available and common treatments, such as antireflective multilayer coating, colored multilayer coating, and mirror coating, can cause a significant reduction in the amount of UVA radiation of the sunlight spectrum being absorbed. Therefore, the photochromic lenses treated with the coating are less active than the photochromic lens that has not been treated with the coating.
The primary object of the present invention is to provide an optical-quality multi-layer treatment for use with a photochromic lens while sufficiently maintaining the photochromic activity of the lens. Other objects and features of the present invention will become apparent from the following detailed description, considered in conjunction with the accompanying examples. It is to be understood, however, that the examples are designed solely for the purpose of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.